In wood pulping which utilizes the kraft and/or sodium sulfite processes, the sulfur compounds in the pulping liquor react to form a number of malodorous organic sulfur compounds as well as hydrogen sulfide and sulfur dioxide. These chemicals are generally in the form of gases and escape to the atmosphere at various stages in the chemical recovery process, particularly in the flue gas from black liquor recovery boilers, from direct contact evaporators, from lime kilns, or from digester and multiple effect evaporator vents.
A substantial amount of the organic sulfur compounds in these gases comprise methyl mercaptan and dimethyl sulfide. Other organic sulfur compounds found in lesser proportion include ethyl sulfide, dimethyl disulfide, isopropyl mercaptan, n-propyl mercaptan and butyl mercaptan. A substantial amount of hydrogen sulfide and sulfur dioxide can also be present in the gases.
In all of these various sulfur compounds, with the exception of sulfur dioxide, the sulfur compounds are formed in the reduction stage of the recovery boiler and will be collectively referred to as total reduced sulfur gases, or TRS, in this specification.
Total reduced sulfur (TRS) gas emissions remain the most serious problem facing the majority of pulp mills when combinations of sodium and sulfur are used as cooking chemicals. The problem is most serious in older kraft mills where sodium sulfide is produced by reduction burning in the recovery boiler and then used in the cooking liquor. ln mills using sodium sulfite as the cooking liquor, sodium sulfide is also produced by reduction burning. TRS gas emissions from the recovery boiler can be a problem in sodium sulfite pulping, as well as in liquor processing up to the point where the sulfides are completely removed from the cooking liquor in the recovery system.
If not adequately controlled, up to 21 pounds of TRS gases are released per ton of pulp produced by the kraft pulp mill. Millions of dollars have been spent by many mills without meeting the recommended limits of government regulations. Due to the extremely low threshold odor in parts per billion of the TRS gases released in kraft pulping, an odor problem remains even when these recommended limits for older mills are complied with.
The odor threshold in air for the average human nose for the TRS gas released in parts per billion as compared to sulfur dioxide is shown below:
______________________________________ Sulfur Dioxide 1000-5000 ppb Hydrogen Sulfide 0.9-8.5 ppb Methyl Mercaptan 0.6-40.0 ppb Dimethyl Sulfide 0.1-3.6 ppb ______________________________________
In kraft pulping, recovery of pulping chemicals following cooking in the digester may be done in various places by a number of processes, including:
1. Pulp washing and spent black liquor collection.
2. Black liquor multiple effect steam evaporation.
3. Black liquor oxidation before and after multiple effect steam evaporation.
4. Black liquor direct contact evaporation with recovery boiler flue gas.
5. Reduction burning of the black liquor in the recovery boiler producing green liquor containing sodium sulfide, sodium carbonate and some sodium hydroxide.
6. Clarification and causticizing the green liquor to white liquor, converting the sodium carbonate to sodium hydroxide and a portion of the sodium sulfide to sodium hydrosulfide. Weak wash liquor is also produced in the causticizing process and contains a high percentage of sodium hydroxide.
7. Lime kiln burning to recover calcium for reuse in causticizing.
8. Cooking the wood chips in a digester with white liquor producing pulp and black liquor.
The TRS gas emissions during kraft pulping may come from the following sources:
Pulp washing vents PA1 Multiple effect steam evaporator vents PA1 Black liquor oxidation PA1 Black liquor direct contact evaporation PA1 Burning the black liquor in the recovery boiler PA1 Green liquor dissolving tank vent PA1 Lime kiln PA1 Digester relief and blow vents PA1 Temperature recovery vents PA1 Tall oil plant vents PA1 Condensate stripper vents PA1 1. INCREASED GAS VELOCITY. Due to the slow absorption rate of carbon dioxide, most absorption towers require low gas velocities in the range under 100 feet per minute. As gas velocities increase the carbon dioxide absorption rate is substantially reduced. Depending on other conditions in the absorber, carbon dioxide absorption can be sufficiently reduced by increasing the gas velocity to 250 foot per minute. Such velocity will provide the desired minimum carbon dioxide absorption requirements of this invention. Higher velocities in the range of 400 feet per minute can minimize absorption further, even when other conditions, described later, favor carbon dioxide absorption. Velocities above 400 feet per minute and up to 1,500 feet per minute can also be used but are restricted by the higher pressure drops encountered, and increase power requirements. To minimize pressure drop and allow high flue gas velocities an open type packing is preferred. Packing suppliers designate packing pressure drop characteristics by an Eckert packing factor (F.sub.p) determined experimentally. The Eckert packing factor is described by Perry & Chilton, Chemical Engineering Handbook, p 18-22, 5th Edition (1973) and by Eckert, Chemical Engineering Progress, 66(3), 39(1970). For the purpose of this invention an Eckert packing factor (F.sub.p)=40 or under is recommended. Eckert packing factors are ordinarily above (F.sub.p)=5. PA1 2. HIGHER ABSORPTION TEMPERATURES. Carbon dioxide absorption is most favorable at scrubbing temperatures below 40.degree. C. At scrubbing temperatures above 60.degree. C. absorption rates for carbon dioxide are substantially reduced with relatively little effect on the absorption of the sulfides. Above 100.degree. C. scaling on the packing can be encountered. Scrubbing temperatures in the 60.degree. C. to 100.degree. C. range are recommended to minimize carbon dioxide absorption and avoid scaling problems. PA1 3. SHORTER RETENTION TIME FOR SCRUBBING SOLUTION. The absorption of carbon dioxide requires a long liquid retention or passage time of the aqueous scrubbing solution for absorption. Towers with trays designed for liquid retention are ordinarily used for carbon dioxide absorption to provide this desired liquid retention time. One means of minimizing carbon dioxide absorption and the formation of sodium bicarbonate is by limiting the liquid passage time of the aqueous scrubbing solution through the absorber to the 10 second to 120 second range. PA1 4. LOWER PACKING HEIGHT. When packed towers are used for carbon dioxide absorption and the formation of sodium bicarbonate, very high or multiple towers are required. The absorption of the TRS requires less packing height and depends on the degree of TRS removal required. Limiting the packing height to the range of 3 feet to 16 feet is recommended to reduce carbon dioxide absorption and limit the formation of sodium bicarbonate. PA1 5. REDUCED RECIRCULATION. To provide the contact needed for adequate TRS removal and limit the quantity of green liquor used, some recirculation of the green liquor is ordinarily required. The quantity of recirculation required will depend on the degree of TRS absorption required, the amount of green liquor available and the pressure drop restrictions based on gas velocities, the packing selected and the packing height. To minimize carbon dioxide absorption and the formation of sodium bicarbonate the recirculation should be kept to a minimum and held in the range of 1,000 to 15,000 pounds of liquor recirculation per square foot of the cross sectional area of the packing perpendicular to the direction of the flue gas flow. PA1 6. MIXING RECIRCULATED GREEN LIQUOR WITH ORIGINAL SUPPLY. Thorough mixing is recommended between the green liquor circulated back to the absorber and the green liquor supply by running the combined flows through the circulating pump or a mixer. This mixing causes any sodium bicarbonate contained in the circulated green liquor to react with the sodium sulfide and/or sodium hydroxide contained in the green liquor supply. The sodium bicarbonate in the recirculated portion is thereby consumed before being reused for scrubbing. This precaution prevents the reaction of sodium bicarbonate and sodium hydrosulfide that would release undesirable hydrogen sulfide at the top of the tower. PA1 8. MAINTAINING HIGH pH. The pH of the green liquor supplied to the absorber is controlled in the 10.7 to 13.0 range by controlling the green liquor feed rate to the absorption system. This allows a limited amount of green liquor to be used and provides means of handling surges of TRS content from sources such as a batch digester blow. A surge of TRS gases will lower the pH of the circulating liquor and requires an increase in green liquor supply in order to absorb the additional TRS gases. PA1 9. DIRECT CONTACT EVAPORATION OF GREEN LIQUOR. Direct contact evaporation of the green liquor may also be provided following its use for absorption. This permits a lower concentration of green liquor to be used for absorption. This also increases the volume of the green liquor used for scrubbing and reduces the ratio of circulated liquor to supply liquor. By cooling the flue gas below 100.degree. C., scaling problems in the packing are avoided.
Present methods of TRS control include:
1. The use of a very expensive recovery boiler design with limited black liquor burning rates. This method is applicable only to new or replacement installations and is applicable only to the TRS gases released by the recovery boiler.
2. Elimination of the black liquor direct contact evaporator and replacement with a more expensive evaporator using steam. This method adds substantially to the energy requirements and operating costs.
3. Oxidation of the black liquor before and after multiple effect steam evaporation to reduce the emission from the direct contact evaporator. TRS emissions are also encountered from the black liquor oxidation when air is utilized. The oxidation method is high in capital and operating costs.
4. Incineration of TRS gases from concentrated sources with auxiliary fuel forming sulfur dioxide. This method is high in capital and energy costs.
5. Incineration of TRS gases from concentrated sources in the lime kiln to form sulfur dioxide. This method introduces serious hazards due to storage requirements of the digester blow gases and due to the backfires and explosions often encountered in the lime kiln burning. In practice lime kiln operations have often been seriously disrupted due to the inherent operating difficulties. Introduction of The TRS gas into the lime kiln also contributes to a sulfur emission problem.
6. Weak sources of TRS gases are also fed back to the air supply of the recovery boiler or the air supply of the lime kiln for incineration to form sulfur dioxide.
While many of these methods help to reduce the odor threshold problem by burning the TRS gases, they unfortunately may contribute to the nations acid rain problem by producing sulfur dioxide that is vented to the atmosphere.
ln a simplified version of chemical recovery of sodium sulfite from spent liquor in sodium sulfite pulping, the green liquor containing sodium sulfide and sodium carbonate is carbonated with carbon dioxide forming sodium bicarbonate and stripping the sulfides as hydrogen sulfide. The hydrogen sulfide is then burned to form sulfur dioxide which is subsequently absorbed by sodium carbonate and/or sodium sulfite to form reconstituted sodium sulfite and/or sodium bisulfite for reuse in cooking. Sodium carbonate can also be causticized to form sodium hydroxide for reuse in cooking.
In the recovery of sodium sulfite, if the conversion of the sodium sulfide to sodium sulfite is complete, the TRS gas emissions are limited to the recovery boiler and green liquor or smelt dissolving tank. If conversion is not complete, TRS gas emission problems are encountered in other locations similar to those encountered in the kraft process. Methods for complete conversion of the sulfides to sulfite were described previously in Farin, U.S. Pat. No. 4,148,684 issued Apr. 10, 1979.
In the initial stages of carbonation in most processes for sodium sulfite recovery, a portion of the recovery boiler flue gas is used for precarbonation of the green liquor.
Examples of precarbonation using a portion of the recovery boiler flue gas are described in the Tampella process, U.S. Pat. No. 3,508,863 to Kimminki et. al., and Sivola method, Finnish Pat. No. 27,478.
In Anderson, U.S. Pat. No. 3,826,710 a portion of the hydrogen sulfide stripped from the green liquor and containing carbon dioxide is used for precarbonation in a countercurrent tower. Absorption of the hydrogen sulfide is obtained in this countercurrent system even though a high percentage of the sodium carbonate contained is carbonated to sodium bicarbonate.
In these prior art applications the purpose of precarbonation by scrubbing the green liquor with the flue gas is to convert the sodium sulfide to sodium hydrosulfide and a portion of the sodium carbonate to sodium bicarbonate. The conversion of sodium sulfide to sodium hydrosulfide occurs by absorbing either carbon dioxide or hydrogen sulfide. The hydrogen sulfide is absorbed near the top of the tower where there is a high percentage of sodium sulfide and a minimum of sodium bicarbonate.
When all the sodium sulfide has been converted to sodium hydrosulfide only carbon dioxide can be absorbed converting the sodium carbonate to sodium bicarbonate. When sodium bicarbonate is formed under these conditions it will react with the sodium hydrosulfide to form sodium carbonate and release hydrogen sulfide.
The purpose of using a portion of the flue gas for precarbonation is to use this source of carbon dioxide to build up the sodium bicarbonate content without causing a hydrogen sulfide emission problem. As the sodium bicarbonate content is increased at the bottom of the precarbonation column it reacts with the sodium hydrosulfide releasing hydrogen sulfide near the bottom of the column. This hydrogen sulfide is reabsorbed by sodium sulfide near the top of the column.
The precarbonation installations are limited to countercurrent towers without recirculation of the green liquor. lf the high sodium bicarbonate concentrations formed in the bottom of the column were recycled to the top, large quantities of hydrogen sulfide would be released to the atmosphere, which is undesirable.
Prior to this invention green liquor has not been successfully used as the means for absorption of the sulfides from the kraft recovery boiler. Using present methods of scrubbing the flue gas with green liquor, the green liquor absorbs carbon dioxide and converts sodium carbonate to sodium bicarbonate. Sodium bicarbonate in green liquor is detrimental to kraft recovery as it doubles the causticizing chemical requirements as well as the lime kiln capacity needed over that required for sodium carbonate. In addition the large volume of flue gas generated in black liquor reduction burning requires a large quantity of green liquor for satisfactory contact and absorption. The green liquor quantity is limited by the quantity of sodium sulfide and sodium carbonate produced and the high concentrations needed for conversion to white liquor and cooking in the digester. To provide sufficient contact for TRS absorption recirculation of the green liquor would ordinarily be required. Sodium bicarbonate contained in the bottom of the column and recycled to the top would react with the sodium hydrosulfide and release hydrogen sulfide to the atmosphere.
Prior to this invention the scrubbing liquor ordinarily recommended for TRS absorption from kraft mill flue gas was either white liquor or weak wash liquor from the causticizing process that contained a high percentage of sodium hydroxide. Pure sodium hydroxide solutions have also been recommended. U.S. Pat. No. 3,431,165 Buxton is an example of TRS scrubbing using weak wash liquor. The sodium hydroxide contained in these scrubbing liquors also absorbs the TRS gases forming sodium sulfide. Unfortunately the sodium hydroxide in these liquors that has been converted from sodium carbonate to sodium hydroxide by the expensive causticizing process is carbonated back to sodium carbonate and to sodium bicarbonate by the absorption of carbon dioxide. It then requires recausticizing of the sodium carbonate and sodium bicarbonate back to sodium hydroxide for use as white liquor. This adds subsantially to the causticizing equipment requirements and causticizing costs.
The white liquor or weak wash containing a high percentage of sodium hydroxide along with sodium sulfide and sodium carbonate can be used for TRS absorption in accordance with this invention. Green liquor may also contain a minimum of sodium hydroxide in kraft and alkaline sodium sulfite pulping applications. The sodium hydroxide content aids in the absorption of TRS gases but is not essential for TRS absorption in accordance with this invention. The use of green liquor is preferred as it contains a minimum of sodium hydroxide and thereby minimizes recausticizing costs. When the sodium hydroxide is allowed to carbonate to sodium bicarbonate, the reaction with sodium hydrosulfide releasing hydrogen sulfide is also encountered thereby decreasing the TRS scrubbing efficiency.
It is the purpose of this invention to provide an economical method for absorption of total reduced sulfur (TRS) gases from the recovery boiler, direct contact evaporator, lime kiln and other concentrated sources using liquor containing sodium sulfide and sodium carbonate by limiting the formation of sodium bicarbonate. It is also the purpose of this invention to limit the formation of sodium bicarbonate to permit recirculation of the green liquor and improve absorption efficiency. Another purpose of this invention is to provide economical methods for absorption of the sulfur dioxide formed in the pulping process and by the oxidation of the TRS gases.